From rebull@ipac.caltech.edu Mon Aug 29 12:20:52 2011
Date: Mon, 29 Aug 2011 12:20:47 -0700 (PDT)
Subject: untangling things


hi -

OK... before I start reading your mails from the last few days (which may 
take me a bit), let me return to the 'big picture' for a bit. Whenever 
tackling a big problem, I like to review both "What do we know" and "What 
are we trying to find".  If this email is useful, I can copy this into the 
Wiki later.

Remember the first page of the attached? This is a Venn diagram I made for 
you on Friday of your visit that was meant to represent the relationship 
among all the various source lists we were considering. At that point in 
the process, we had already consolidated things into:
- All "bright enough" sources covered by the Spitzer maps (a conceptual 
list only)
- Sources in my catalog of extractions (I provided this)
- Sources in this general direction studied by anyone else, ever (the 
majority of those reported are also YSOs, but not all of them)

out of those sets, our scientific goals mean that we are aiming for a 
discussion of:
- YSO candidates we select from IR excess
- YSOs that others identify that do not appear to have an IR excess.

The first page in the attached was even trying to correctly represent the 
relative sizes of the circles in that "all bright enough sources" ought to 
be darn close to "sources in my catalog" and that there will be some 
"sources in this general direction..." not covered by the maps and some of 
those without IR excesses.


OK, so NOW you are going back to revisit the list of "Sources in this 
general direction studied by anyone else, ever" because even during your 
visit, we had identified some issues with that list.

On the second page of the attached, I tried to spatially represent the 
concepts behind what we're doing now, but I admit the circles are not as 
carefully constructed/laid out as the first page!

Each of these 5 papers (Ogura et al 2002, Gregorio Hetem et al. 2009, 
Chauhan et al 2009, Shevchenki et al 1999 and Wiramihardja et al 1986) 
looked in the direction of BRC27. Surely, then, they saw some of the same 
sources as each other, and as what we are seeing. For example, the 
Gregorio-Hetem survey covered a HUGE area, and we care only about a part 
of it. Ogura saw some of the same sources that Gregorio-Hetem did, but not 
all of them -- they were not covering the same areas, but even within the 
same area, they did not see the same sources, because one survey was x-ray 
driven, and one was Halpha driven. They *will* see different sources, 
because each survey is not infinitely deep -- the sensitivity of the 
surveys is limited, and as such will not see every source in this 
direction. Same for each other pair of papers, and our survey.

The goal here is to construct a list that is as clean as possible for each 
of the objects that these other folks studied, identifying which objects 
are truly the same between surveys, and identifying which of these objects 
are ones that those authors thought were young stars.  We also want to 
carry along each of the relevant bits of information that these other 
authors provided -- you already know that the SED is a lot easier to 
identify as clearly a young object or a contaminant if there is optical 
data, so if the other authors reported any optical measurements, we should 
keep track of those and tie them to the correct object in our analysis. 
We should also make note of any spectral types or other relevant 
information.  The "money question" (page 3 of the attached) is thus:
- Which objects from paper x are also seen in paper y?
and then, the next step is
- Which of these objects are seen in the IRAC data?



This would be an easy task if:
- everyone provided their original images, either as a figure or as a fits 
file
- everyone worked in the same coordinate system, by which i mean not just 
"J2000" vs "B1950" but "J2000 tied to 2MASS" as opposed to "J2000 tied to 
the pulsars seen by NRAO" or "J2000 as calibrated as best I can based on 
the HST Guide Stars I happen to see in my image" (the latter of which is 
what we are likely to have with the Haleakela data, by the way).
- the objects were all greater than 5 arcseconds apart from each other on 
the sky, such that each source that is detected was cleanly and uniquely 
detected in each survey.
- and, of course, that we were guaranteed a match between surveys.

Working backwards up that list...

We've already talked above about how we are not guaranteed a match between 
surveys, because stars are different brightnesses at different bands, and 
because the surveys have limited sensitivity.

You know from staring at the images that there are plenty of sources that 
are very close together. Even among just the YSO candidates, some are very 
close to each other, closer than 5 arcseconds.

If we had fabulous coordinates for everything, we could let the computer 
match them all up and not worry about it. But we don't. And it's not just 
a matter of precessing the B1950 ones to J2000 ones either - there are 
inherent errors in those old coordinates which will not necessarily 
smoothly map into a clean match to other objects.

If we had images, we could line them up by eye and identify the same 
objects in each frame. I don't necessarily mean "line them up in ds9" 
since we don't have any fits files. I mean, by comparison between images 
they publish and images to which you have access (IRAC, 2MASS, POSS), you 
can identify the objects. This is what we are going to have to retreat to, 
in the tough cases.

Here are some notes on these 5 papers, in no particular order:

Gregorio-Hetem - finding charts for a few complex fields, which may or may 
not be in our region. J2000 coordinates tied to 2mass, though, so less 
worried about these. note that their discussion includes this very 
conceptual problem -- they are trying to tie X-ray sources to optical 
sources.
Wiramihardja - no finding charts; a few very coarse-scale ones, one with 
just YSOs. Not a lot of help here. Also just 1950 coordinates read off a 
photographic plate. Likely to be large and unsystematic errors.
Shevchenko - finding chart provided (fig 1). 1950 coordinates. big field 
(bigger than the region we care about).
Ogura - finding chart provided; J2000 coordinates but not necessarily 
tied to 2MASS. 
Chauhan - no real finding charts provided; there are a few coarse-scale 
ones. J2000 coordinates tied to 2mass, though, so less worried about 
these.

For each of these 5 papers, we obtained a machine-readable (read as 
"plain text file that the computer can parse into individual numbers 
rather than images of numbers") version of the relevant data tables. This 
was either:
- obtained from the journal itself, in which case the data table is much 
longer than we need
- obtained from what diane did (she followed what i was telling her to 
do), where she typed in the coordinates of the objects in our fields from 
these older papers and attempted to get updated coordinates.
These tables are at the bottom of this page here:
http://coolwiki.ipac.caltech.edu/index.php/BRC_Spring_work
Look down under "Literature known YSOs" and then under "These papers in 
order of priority" and then each paper's listing has a link to files, 
e.g., "File:Chauhan-table3.txt".

What Diane did on my advice (see the YouTube video on that page) SHOULD 
work in MOST cases but did not work in every case; more on this momentarily.
What she did was :
-type the 1950 coordinates into the twomass point source archive, making 
sure that it knows that it is 1950 coordinates.
-look at what comes back, and take the closest bright object.

Then we had 5 lists of UPDATED, HIGH QUALITY coordinates, one per paper, 
and we let the computer run through the list, finding the matches between 
papers. I then generated one file that purported to have one line per 
literature object, with all the relevant data on that line. But we were 
able to identify problems with this, in that a few sources during this 
process were tied to the same object, or identified in other ways as 
duplicates or incorrect matches.

What I recommended before that you (plural) do to get updated coordinates 
for targets works ON THE ASSUMPTION THAT THERE ARE NOT "TOO MANY" SOURCES 
NEARBY, AND THAT THE PRIOR SURVEYS, BEING SHALLOW, ARE MOST LIKELY TO 
MATCH TO THE BRIGHT 2MASS SOURCE. As I say, this should work in MOST cases 
but not ALL of them. In some cases, two distinct sources reported, e.g., 
two sources in Ogura were matched to the same 2MASS source during this 
process, and thus matched to the same IRAC source when we let the computer 
merge the lists.

To untangle this, we need to go back, possibly just to the duplicate 
sources, possibly through all of them if you can't identify with certainty 
which are the duplicates, and make really sure we have obtained the best 
possible coordinates. If the original authors are reporting more than one 
source within one paper, we should not consolidate them into one unless we 
are REALLY sure that the original authors were wrong. (This does happen; 
see my North American Nebula paper for examples.)  Within each paper, this 
is what I would do:

-Go back and do the 2MASS archive search again
(Go here: http://irsa.ipac.caltech.edu/applications/Gator/
pick 2MASS then on the next page pick 2mass point source catalog (PSC), 
then use that search page, making sure to specify which coordinate 
system in which you are working.)
-Look at the sources returned. Is there just one within 5-10 arcsec? 
that's probably your match; go on to the next source. If there is more 
than one, is there just one BRIGHT one within 5-10 arcsec? Is there 
another source from that same paper within 5-10 arcsec? If the answer to 
either of those questions is yes, go on to the next step.
- Go get the images in another window.  Compare the images (if provided) 
from the original papers.  We can use Finder Chart 
(http://irsa.ipac.caltech.edu/applications/FinderChart/) to get DSS and 
2MASS images of the region where there is a confusing match.  Compare this 
to the images provided in the papers. Identify which object is the 
one(s) in the paper.
- With that information, then go back to your PSC, and then use the 2MASS 
point source catalog to find the actual high-precision coordinates of that 
specific dot you have identified in the images as the match to the 
literature object.


Then I (or you) can take the updated high-precision lists of coordinates, 
merge them together, and come up with a new-and-improved list of all the 
previously identified objects in this region, one line per object with all 
the relevant information.

Then we can compare this list to our detections in IRAC. There will be 
some with IR excesses, and some without IR excesses.

Does this all make more sense?

I will work on catching up on all the emails as soon as I can. I was out 
ThFr last week, and the amount of backlogged mail was ... scary.

cheers,
Luisa





Dr. Luisa Rebull            Research Scientist, Spitzer Science Center
Caltech M/S 220-6           voice 626-395-4565
1200 E. California Blvd.
Pasadena, CA 91125

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